Thermoelastic equilibrium of fluid inclusion–host mineral systems under homogenization measurement

Abstract

Microthermometric data of fluid inclusions can elucidate the physicochemical properties of lithospheric fluids, but the inclusions must satisfy several criteria to yield proper fluid information. One is the 'constant volume criterion': the inclusion volume must remain constant after being trapped. However, volume changes of fluid inclusions occur in nature during emplacement underground or uplifting to the surface. They can also occur during sample preparation or data collection in the laboratory. Therefore, thermoelastic deformation of the crystal lattice surrounding fluid inclusions during experiments might increase uncertainty about microthermometric data. Herein, we introduce and assess a method using the equation of state of pure CO 2 and thermoelastic equilibrium between a fluid inclusion and a host mineral to estimate fluid inclusion volume changes accurately during measurements of homogenization temperature. The estimation is valid if the inclusion and host are isotropically elastic, concentric spheres. Subsequently, we extended the equation of state to a more comprehensive Redlich and Kwong equation of state, which is applicable to more complicated fluid systems. Furthermore, for accurate treatment of the elastic effects involving an elastically anisotropic host mineral, we propose the method's application to an anisotropic host mineral. If physical properties of the host mineral and the molar volume of the fluid inclusion at an arbitrary temperature are known, then one can use this method for accurate estimation of the molar volume at a given temperature. This microthermometric data based method can accurately elucidate characteristics of the crust and mantle-fluid activity.

Water-rock interaction in the subduction zones: Perspective from metamorphic rocks

“…．しかしながら，それらの研究の多 くは岩石側の全岩，或いは鉱物の局所的な化学組成変化か ら流体の活動を間接的に制約するに留まっており，地下深 部で生じていた流体の化学組成を直接制約する例は少ない。 鉱物中に含まれる流体包有物は，包有物形成時の流体そ のものであり，変成岩中の流体包有物は変成作用で放出さ れる流体の特性を直接的に調べる格好の材料と言える (Scambelluri and Philippot, 2001;Scambelluri et al, 2004)。また，流体包有物の組織自体が温度圧力変化など のテクトニックな履歴を示すため，岩石の P T 史解析に も有用である (P âecher, 1981;Boullier et al, 1991) り，包有物のバルク密度の"焼き鈍し"が起きた結果であ ろうと思われる。減温減圧過程での流体包有物の密度変化 については鉱物の弾性変形からのモデル化もされており (Hagiwara et al, 2019)，今後研究が進むことで包有物密 度情報の有効活用が期待される。 話を化学組成分析に戻すと，流体包有物の化学組成を凍 結させて直接分析する試みは，クライオ SEM での溶存元 素組成分析 (Ayora and Fontarnau, 1990)，クライオ SIMS での同位体組成分析 (Yurimoto et al, 2014)…”